1. Department of Chemistry, Rice University, Houston, Texas 77005, United States
2. Center for Theoretical Biological Physics, Rice University, Houston, Texas 77005, United States
3. Mathematical and Statistical Computing Laboratory, Division of Computational Bioscience, Center for Information Technology, National Institutes of Health, Bethesda, Maryland 20892, United States
4. Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, United States
5. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, United States